Used in industrial measurements technology, especially also in connection with control and monitoring of automated manufacturing processes, for ascertaining mass flow rates of media such as liquids and/or gases flowing in a process line, for example, a pipeline, are often Coriolis mass flow measuring devices formed by means of a measuring device electronics as well as a measuring transducer of vibration type electrically connected with said measuring device electronics. Coriolis mass flow measuring devices have been known for a long time and have proven themselves in industrial use. Examples of such measuring devices are described e.g. in US-A 2007/0119264, US-A 2010/0257943, US-A 2011/0265580, U.S. Pat. No. 5,287,754, U.S. Pat. No. 5,291,792, U.S. Pat. No. 5,531,126, U.S. Pat. No. 5,602,345, U.S. Pat. No. 5,731,527, U.S. Pat. No. 5,796,010, U.S. Pat. No. 5,796,011, U.S. Pat. No. 5,945,609, U.S. Pat. No. 6,006,609, U.S. Pat. No. 6,092,429, U.S. Pat. No. 6,223,605, U.S. Pat. No. 6,311,136, U.S. Pat. No. 6,513,393, U.S. Pat. No. 6,840,109, U.S. Pat. No. 6,920,798, U.S. Pat. No. 7,017,424, U.S. Pat. No. 7,077,014, U.S. Pat. No. 7,325,462, WO A 01/02 816, and WO A 99/40 394.
Each of the therein disclosed measuring transducers comprises at least one, essentially straight or at least sectionally curved, measuring tube for conveying the medium. The measuring tube is additionally especially adapted to be flowed through by the medium and during that to be caused to vibrate in such a manner that it executes about an imaginary oscillation axis, most often an axis parallel to an imaginary longitudinal axis of the measuring transducer, bending oscillations—herein referred to as wanted oscillations or also wanted mode—having a wanted frequency corresponding to a resonant frequency of the measuring transducer. The wanted oscillations of the measuring tube serve in the case of Coriolis mass flow measuring devices, among other things, for inducing in the flowing medium Coriolis forces dependent on the instantaneous mass flow rate. These Coriolis forces, in turn, bring about a measurement effect dependent on the mass flow rate, namely Coriolis oscillations with the wanted frequency superimposed on the wanted oscillations. In the case of measuring transducers with curved measuring tubes, e.g. formed U-, V- or -like, selected for the wanted oscillations is usually a fundamental mode of a bending oscillation or an oscillation form corresponding to a next higher ordered, symmetric, bending oscillation mode, in the case of which oscillation form the at least one measuring tube moves about the imaginary oscillation axis in a pendulum like manner like the oscillations of a cantilever clamped at one end. The Coriolis oscillations resulting therefrom correspond to that eigenoscillation form—at times, also referenced as twist mode—, in the case of which the measuring tube executes rotary oscillations about an imaginary rotation axis directed perpendicularly to the imaginary oscillation axis, most often also parallel to an imaginary vertical axis of the measuring transducer. In the case of measuring transducers with straight measuring tubes, in contrast, selected for the purpose of producing Coriolis forces is often such a wanted mode, in the case of which the measuring tube executes, at least partially, bending oscillations essentially in a single imaginary plane of oscillation, so that the Coriolis oscillations are embodied accordingly as bending oscillations, which are coplanar with the wanted oscillations. Due to the Coriolis oscillations, there exists between inlet-side and outlet-side, oscillatory movements of the vibrating measuring tube a travel time, respectively phase difference, dependent on the mass flow rate, and, consequently, usable as measurement effect for the mass flow measurement. Since the wanted frequency is, especially, also dependent on the instantaneous density of the medium, besides the mass flow rate, supplementally also the density of flowing media can be measured by means of market-usual Coriolis mass flow measuring devices. Furthermore, it is also possible, such as disclosed, among other things, in the initially mentioned US-A 2011/0265580, by means of such measuring transducer of vibration type, consequently therewith formed Coriolis mass flow measuring devices, directly to measure viscosity of the flowing medium, for example, based on an excitation power required for exciting, respectively maintaining, the wanted oscillations.
In the case of measuring transducers with two measuring tubes, these are most often connected into the process line via a distributor piece extending on the inlet-side between the measuring tubes and an inlet-side, connecting flange as well as via a distributor piece extending on the outlet-side between the measuring tubes and an outlet-side, connecting flange. In the case of measuring transducers with a single measuring tube, the latter communicates most often with the process line via a connecting tube on the inlet-side as well as via a connecting tube on the outlet-side. Furthermore, measuring transducers with a single measuring tube include at least one counteroscillator of one piece or a plurality of parts, for example, a tube-, box- or plate-shaped, counteroscillator, which is coupled with the measuring tube on the inlet-side to form a first coupling zone and with the measuring tube on the outlet-side to form a second coupling zone. During operation, the counteroscillator essentially rests or oscillates opposite equally relative to the measuring tube thus with equal frequency and opposite phase. The inner part of the measuring transducer formed by means of measuring tube and counteroscillator is most often held in a protective measuring transducer housing alone by means of the two connecting tubes, via which the measuring tube communicates during operation with the process line. Especially, this assembly is accomplished in a manner enabling oscillations of the inner part relative to measuring transducer housing. In the case of the measuring transducers disclosed, for example, in U.S. Pat. No. 5,291,792, U.S. Pat. No. 5,796,010, U.S. Pat. No. 5,945,609, U.S. Pat. No. 7,077,014, US-A 2007/0119264, WO A 01/02 816 or and WO A 99/40 394 having a single, essentially straight, measuring tube, the latter and the counteroscillator are, such as quite usual in the case of conventional measuring transducers, oriented essentially coaxially with one another, in that the counteroscillator is embodied as an essentially straight, hollow cylinder and is so arranged in the measuring transducer that the measuring tube is at least partially jacketed by the counteroscillator. Used as materials for such counteroscillators, especially also in the case of application of titanium, tantalum or zirconium for the measuring tube, are, most often, comparatively cost effective steel types, such as, for instance, structural steel or free machining steel.
For active exciting, respectively maintaining, of the wanted oscillations, measuring transducers of vibration type have, additionally, an exciter mechanism formed by means of at least one electromechanical oscillation exciter acting during operation differentially on the at least one measuring tube and the, in given cases, present counteroscillator, respectively the, in given cases, present, other measuring tube. The oscillation exciter, most often an electrodynamic oscillation exciter, serves, driven by an electrical exciter signal, for example, having a controlled electrical current, generated by the mentioned measuring device electronics and correspondingly conditioned, especially to convert an electrical excitation power fed therewith into a corresponding drive force effecting the wanted oscillations, for example, also in such a manner that the wanted oscillations have a constant oscillation amplitude. Oscillation exciters of usually marketed measuring transducers of vibration type are typically constructed as a kind of oscillatory coil, namely, in the case of measuring transducers with a measuring tube and a thereto coupled counteroscillator, formed of a magnet coil most often affixed to the latter as well as a permanent magnet serving as magnet armature correspondingly affixed to the measuring tube to be moved and interacting with the at least one magnet coil. The permanent magnet and the magnet coil are, in such case, usually so oriented, that they extend essentially coaxially with respect to one another. Additionally in the case of conventional measuring transducers, the oscillation exciter is most often so embodied and placed that it acts essentially centrally on the at least one measuring tube. Alternatively to an exciter mechanism formed by means of oscillation exciters acting centrally and directly on the measuring tube, it is also an option, such as set forth, among other things, in the initially mentioned U.S. Pat. No. 6,092,429, for example, also to use exciter mechanisms formed by means of two oscillation exciters affixed not in the center of the measuring tube, but, instead, rather on the inlet, respectively outlet, sides thereof, or, such as provided, among other things, in U.S. Pat. No. 6,223,605 or U.S. Pat. No. 5,531,126, for example, also exciter mechanisms formed by means of an oscillation exciter acting between the, in given cases, present counteroscillator and the measuring transducer housing.
For registering inlet-side, respectively outlet-side, oscillations of the at least one measuring tube, not least of all of its oscillations with the wanted frequency, and for producing the electrical oscillation signals influenced for measuring mass flow rate, measuring transducers of the type being discussed have, furthermore, two or more oscillation sensors spaced from one another along the measuring tube. Such oscillation sensors are so adapted that therewith generated oscillation signals led to the measuring device electronics have a wanted signal component with a signal frequency corresponding to the wanted frequency and that the aforementioned travel time, respectively phase difference, is measurable between a wanted signal component of an oscillatory signal representing inlet-side oscillations and a wanted signal component of an oscillatory signal representing outlet-side oscillations. In the case of measuring transducers of usually marketed Coriolis mass flow measuring devices, the oscillation sensors are essentially embodied of equal construction as that of the at least one oscillation exciter, at least, insofar as they work according to the same principle of action, for example, being again of electrodynamic type. Accordingly, the oscillation sensors of such a sensor arrangement are also most often likewise, in each case, formed by means of a permanent magnet affixed to the measuring tube and at least one coil (for example, a coil affixed on the, in given cases, present, other measuring tube or on the, in given cases, present counteroscillator) permeated by a magnetic field of the permanent magnet. As a result of the oscillatory movements of the at least one measuring tube, the coil then experiences, at least at times, an induced measurement voltage.
Both the coil of the at least one oscillation exciter as well as also the coils of the oscillation sensors are additionally electrically connected, in each case by means of a pair of electrical connecting lines, with the measuring device electronics.
In the case of conventional Coriolis mass flow measuring devices, the ascertaining of the mass flow, measured values representing the mass flow rate occurs fundamentally based on a ratio of the phase difference to the wanted frequency multiplied by an earlier calibrated, measuring device specific, calibration factor, plus or minus a zero point of the measuring device corresponding to a measurable phase difference when there is no flow in the measuring tube. Thus, the calculation is based on a linear function of said ratio. At times calibration factor affecting dependencies on density and/or viscosity of the momentary medium or also such dependencies on the momentary pressures reigning in the medium are compensated in such case in measuring operation by means of special measuring and calculating algorithms correspondingly implemented in the measuring device electronics. Experimental investigations on conventional Coriolis mass flow measuring devices have now shown that the aforementioned mathematical approach usually selected for calculating the mass flow, measured values can lead, at times, to increased measurement errors. This is the case surprisingly especially also for measuring transducers having a nominal diameter of less than 50 mm corresponding to the caliber of the connected process line, respectively corresponding to a nominal cross section of less than 2000 mm2 or for measuring transducers with measuring tubes, whose caliber is less than 20 mm, consequently especially in the case of measuring transducers, which usually have a rather high area-normalized sensitivity, defined as the ratio of the phase difference to an area-normalized mass flow rate, especially a mass flow rate referenced to the nominal cross section of the measuring transducer, of greater than 10 rad kg−1·s·mm2. Moreover, it could be detected that increased measuring errors can also happen increasingly in the case of fluid dynamically rather stable mass flow rates, consequently actually uncritical mass flow rates, in the order of magnitude of 50% of the respective measurement range end value. In other words, an undesired dependence of the accuracy of measurement, with which the mass flow rate is ascertained, on the actual mass flow rate of the medium could be detected. Moreover, it could be detected that said measuring error additionally also has a certain dependence on the instantaneous density of the medium.